It is common practice in the vacuum furnace industry to have a hot zone which has an enveloping structure. The enveloping structure is in fact an insulation package. The enveloping structure is very often cylindrical in shape with a cavity formed therewithin. Within the cavity the work pieces (which are going to be heat treated) are located. The enveloping structure comprises an outside wall and an inside wall with the inside wall being separted from the outside by a space. Within the separation there are located multilayers of graphite felt. I have found that the multilayers of insulation tend to "break up" after repeated cycles of hot and cold temperatures. The consequence of the "breaking up" of the insulation material has been to have particles of the insulation material pass from the ends of the insulation layers into the hot zone (or other sections of the furnace) and contaminate work pieces or contaminate sections of the furnace. The present device prevents such contamination.